how to modifing existing torus's minor radius
No you can't. Once a primitive is set you cannot change it's properties, and there is no tool that will allow you to just change the inner radius without modifying the outter. Alt + S will just shrink or fatten the whole mesh. You can however create a new torus and copy location and rotation with the copy object addon to replace the old one. But if you want to be able to go change torus size at all time it might be good to try using a circle curve with some bevel to it instead of a mesh.